1. Field of the Invention
The present invention relates to an image processing system which creates composite image information by embedding, in visible main image information (e.g., a human facial image), another additional sub-information (e.g., security information) in an invisible state, records the created composite image information on a recording medium, and restores the embedded sub-information from the recorded composite image information, and an image processing method and apparatus used in the image processing system.
2. Description of the Related Art
Recently, with the trend toward computerization of information and the proliferation of the Internet, increasing importance has been attached to a digital watermarking technique, digital signature technique, and the like to prevent counterfeiting and alteration of images. The digital watermarking technique of embedding additional sub-information (sub-image information) in main image information in an invisible state, in particular, has been provided as a counter-measure against unauthorized copying, counterfeiting, and tampering of a personal authentication medium such as an ID card or a photograph in which copyright information is embedded.
For example, Jpn. Pat. Appln. KOKAI Publication No. 9-248935 discloses en digital watermark insertion method of embedding data in image data output onto printed matter by using the characteristics of high spatial frequency components and color difference components which are difficult for man to perceive. Jpn. Pat. Appln. KOKAI Publication No. 2001-268346 discloses a printing apparatus for digital watermarks that can be recognized through optical filters.
In Jpn. Pat. Appln. KOKAI Publication No. 9-248935, there is a description about a method of predicting the degree of deterioration and increasing the strength of embedding in accordance with the predicted degree. This method, however, increases the risk of disclosing sub-information.
A sublimation/thermal transfer recording scheme is generally used to record facial images for personal authentication on various kinds of ID cards such as driver's licenses and personal authentication media typified by membership cards.
In general, in the sublimation/thermal transfer recording scheme, materials that can be dyed with sublimable materials are limited. This scheme can therefore be adapted to only limited recording media. For this reason, the degree of freedom in selecting recording media as personal authentication media on which facial images for personal authentication are recorded is low. As a consequence, easily available media must be selected. This often decreases the security. In addition, sublimable dyes generally have poor image durability, e.g., poor light resistance and poor solvent resistance.
In contrast to this, in a fusion thermal transfer recording scheme, a material having good light resistance can be generally selected as a coloring material. This scheme therefore allows a high degree of freedom of choice regarding recording media. In the scheme, therefore, a high-specialty recording medium can be used. This makes it possible to improve security. The fusion thermal transfer recording scheme, however, uses a dot area gradation method of performing gradation recording by changing the sizes of transferred dots. With this scheme, therefore, it is difficult to realize as high gradation performance as that with the sublimation/thermal transfer recording scheme.
In order to solve this problem, for example, Jpn. Pat. Appln. KOKOKU Publication No. 6-59739 discloses a method of recording transferred dots in a so-called staggered array (this method will be referred to as an alternate driving/recording scheme hereinafter).
Jpn. Pat. Appln. KOKOKU Publication No. 6-59739 discloses a recording method of improving the gradation recording performance in the fusion thermal transfer recording scheme. If, however, facial image information in which watermark information is embedded by using a digital watermarking technique is recorded, data is thinned out in a staggered pattern, resulting in loss of corresponding information. This destroys the digital watermark information.